Identification of a gene in the euryarchaeal Thermococcus species AN1 encoding a protein homologous to the alpha subunit of the eukaryal signal recognition particle (SRP) receptor

We describe here the sequence and transcriptional analysis of a gene from the euryarchaeal Thermococcus species AN1 (DSM 2770) encoding a protein homologous to the alpha subunit of the eukaryal SRP receptor (SR alpha). The AN1 protein is found to share the highest degree of homology with the only other described archaeal SR alpha homolog characterized from the hyperthermophilic crenarchaeal Sulfolobus solfataricus. Sequence analysis of the translation of the AN1 gene reveals the presence of the following previously described domains: an N-terminal alpha domain rick in acidic and basic residues; an X domain and four GTP binding motifs (G1-G4). A putative guanine nucleotide dissociation stimulator binding element is also present. The ANI SR alpha protein would now represent the shortest variant of this expanding family with 329 residues and a predicted molecular weight of 36.3 kDa. A Northern analysis indicates that the ANI SRcr protein gene transcript is present at low levels suggesting that SR alpha is likely to be only a minor cell constituent. The presence of an SRcr homolog in another kingdom within the archaeal domain possessing the full suite of conserved motifs is significant in several respects. It not only supports the monophyletic character of the domain Archaea but suggests that these homologs have similar functions in these organisms and emphasises the ancient origins of the protein export machinery.